The present invention relates to a brake control method and a brake control device, which are used for a braking device provided to a vehicle or the like, the braking device having a function of applying a brake force to a wheel equipped with a tire to put a brake thereon while adjusting the brake force, and more particularly, to a brake control method and a brake control device capable of controlling a brake force to be applied to a wheel without performing frequency analysis.
When a vehicle is making a turn, a tread portion of a tire provided with a slip angle partially slips on a road surface. In other words, the tread portion of the tire provided with a slip angle has a contact region formed of a region in which the tread portion adheres to the road surface (adhesion region) and a region in which the tread portion slips on the road surface (slip region). As the slip angle increases, the adhesion region decreases while the slip region increases. In the end, the whole contact region of the tire is formed only of the slip region, leading to a behavior in which the entire tire slide-skids on the road surface (slide skid behavior).
In general, it is difficult for a driver to detect the slide skid behavior. Further, once the skid behavior has occurred, it is difficult to suppress the skid behavior to return the steering to a controllable state. As a result, the steering of the vehicle goes out of control, often resulting in a car crash.
In view of the above-mentioned problem, there has been conventionally adopted a method in which a vehicle control device such as an anti-lock brake system (ABS) is provided to a vehicle so as to perform brake control of the vehicle, to thereby prevent the slide skid from occurring (see, for example, JP 2003-182476 A and JP 2005-306160 A).
JP 2003-182476 A discloses a vehicle control device capable of estimating the condition of a road surface to which a tire is brought into contact with and a travelling condition of the tire with high accuracy when a vehicle is travelling at a constant speed without any operations such as braking, driving, and steering, to thereby improve the travelling safety of the vehicle.
The vehicle control device according to JP 2003-182476 A includes a road surface condition and tire travelling condition estimating device and a vehicle control means for controlling the travelling condition of a vehicle based on a condition of a road surface and/or a travelling condition of the tire. The road surface condition and tire travelling condition estimating device frequency-analyzes, by a frequency analyzing means, a vibration information signal of a wheel detected by an acceleration sensor attached to a wheel rim to detect a vibration level of the vibration spectrum of the signal. Then, the device obtains the detected vibration level by using a G-table stored in a vibration level storing means, the G-table indicating a relation between the road surface friction coefficient μ and vibration level, to thereby estimate the road surface friction coefficient μ.
JP 2005-306160 A also discloses a vehicle control device capable of extracting high frequency components from information of a vibratory waveform detected by an acceleration sensor and detecting the amplitude of the high frequency components when the tire tread passes by the trailing edge of the tire contact region with a ground, to estimate whether the road surface friction coefficient is large or small by comparing an amount of the amplitude with a threshold value set in advance, to thereby accurately estimate a road surface friction coefficient without frequency-analyzing data of the vibratory waveform.
However, in any one of the vehicle control devices disclosed in JP 2003-182476 A and JP 2005-306160 A, a frequency analysis is performed on a signal detected by the acceleration sensor, by frequency-analyzing the signal by the frequency analyzing means, or by extracting high frequency components from the signal, which leads to a problem that it is impractical because it takes time to start control of the vehicle. To make the device suitable for practical use, the processing rate of the frequency analysis needs to be increased.